1. Field of the Invention
The present invention relates to a color encoder for coding a color video signal and, more particularly, to a circuit with a comb filter for causing different kinds of color difference signals of a line-sequential color difference signal sequence to coincide on each scanning line to produce a composite color video signal.
2. Description of the Prior Art
In an electronic still camera system or similar imaging system, a color video signal is usually recorded on a floppy video disk by adopting a color difference line-sequential system. When such a signal is to be read out of a floppy video disk to produce a composite color video signal which is prescribed by a standard color television system, for example, two different kinds of color difference signals which appear line-sequentially and alternately with each other are so arranged as to coincide on each of consecutive scanning lines by a special procedure. This procedure for the line-by-line coincidence of color difference signals has heretofore been implemented by supplementing a scanning line which lacks one of the two kinds of color difference signals with a color difference signal of the same kind that appeared on the immediately preceding scanning line.
A prior art circuit for effecting the supplement mentioned above includes a delay circuit for delaying color difference signals of a line-sequential signal sequence by one horizontal scanning (1H) period. The delayed color difference signals are individually clamped together with non-delayed color difference signals and then switched in such a manner that color difference signals of one kind appear on one input terminal of an encoder and those of the other kind appear on the other input terminal of the same without exception. The encoder applies balanced modulation to color subcarriers by using the color difference signals which are now coincident with each other on a line basis. The color subcarriers, or color difference signals, that have undergone balanced modulation are routed through a comb filter to a mixer to be combined with a luminance signal, whereby a composite video signal is produced. The comb filter is also implemented by a 1H delay circuit for enhancing the signal-to-noise (S/N) ratio of the color difference signals. Since the color difference signals are delayed by 1H as stated above, the luminance signal has to be delayed by 1H by an exclusive 1H delay circuit so that its time axis may coincide with that of the color difference signals.
As described above, the prior art circuit for the coincidence of color difference signals needs three 1H delay circuits. Moreover, since a line-sequential color difference signal sequence has a narrow frequency band, the delay circuit adapted for the coincidence of two kinds of color difference signals has to be implemented as a charge-coupled device (CCD) delay circuit. A drawback with a CCD delay circuit is that the scattering of transfer charge level particular thereto has to be compensated for by accurately adjusting line by line the levels of the signals which are coincident with each other. Thus, the prior art circuit is complicated in construction and needs adjustment which is troublesome.